There are three well known techniques for dry particle development of a latent, electrostatic image in a non-impact printing device: cascade, magnetic brush and cloud application. In each technique a mix of toner particles, typically formed of ferromagnetic carrier granules that are triboelectrically coupled to carbon-impregnated resin particles, is applied to the latent, electrostatic image. As the toner mix approaches the electrostatic image, the coulomb attraction between the carbon-impregnated resin particles and image overtakes the triboelectrical coupling between the carrier granules and resin particles and causes the carbon-impregnated resin (or similar pigment) to adhere to the image, thus the image is developed. Each technique has advantages and disadvantages which require tradeoffs between speed and performance, as will be discussed presently.
The cascade technique of toner application involves dispensing toner from a bucket or other like container and allowing it to fall under gravitational influence onto the dielectric surface bearing the latent electrostatic image. Commonly, the dispensing container is one of a plurality of such containers arranged in conveyor-like fashion on an endless chain or belt.
The cascade technique is limited in the speed at which it can feed toner to the latent electrostatic image. Specifically, the cascade technique generally employs a complex mechanical movement to transport toner from a source to the dispensing station; e.g. a conveyor of buckets. Moreover, the cascade development technique needs a developing electrode to develop a large solid area. The developing electrode has to be fairly close to the dielectric surface, hence it limits the flow rate of toner particles, therefore limiting the toner speed.
The cascade technique does, have an advantage if the electric field associated with the latent image is weak. In those instances the cascade technique imparts adequate kinetic energy to the triboelectrically coupled toner components to allow them to break free of their mutual attraction when they impact the dielectric surface supporting the latent electrostatic image. As a result, there is an abundance of free resin (or other type pigment) particles to couple a weak latent electrostatic image.
The magnetic brush technique typically employs a cylindrical drum or roller which has a permanent magnet disposed within it. Through each rotation of the drum or roller, toner particles are picked up through the magnetic field associated with the magnet and transported to the dielectric surface supporting the latent, electrostatic image. The magnetic brush technique has the advantage of speed, but also has the disadvantage of imparting little kinetic energy to the triboelectrically coupled particles and requires a relatively strong electrostatic image to assure quality image development.
The cloud technique involves the formation of a cloud-like distribution of toner mix in the area of the dielectric surface supporting the electrostatic image. The cloud technique is suitable for the development of extremely weak latent electrostatic images. The drawback, however, is that resin particles may come to rest on the dielectric surface in uncharged areas causing unwanted spots on the background of the transferred image.
This review of the prior art points out the need for a toning device that provides high speed development, accommodates weak latent, electrostatic images and otherwise avoids limitations attendant to the prior art.